From the FAQ:
Can I delete my account?
We try not to delete entire account histories because that would gut the threads the account had participated in. However, we care about protecting individual users and take care of privacy requests every day, so if we can help, please email hn@ycombinator.com. We don't want anyone to get in trouble from anything they posted to HN. More here. [1]
In summary one can delete their account but they have to specify a reason to clear out all the comments.
[1] - https://news.ycombinator.com/item?id=23623799
